The present invention relates to a polarization element for use in optical communications, optical recording and optical sensors, and a method for manufacturing the polarization element, and more particularly, to a polarization element which includes a fine metal particle having a shape anisotropy, and a method of manufacturing the same.
A fine particle dispersion including a fine particle having a shape anisotropy, deposited and dispersed in a matrix material, has polarization properties, and is utilized for polarization elements. The shape anisotropy of fine particle is obtained by drawing the fine particle dispersion.
A linear polarizing film, which is a polarization element utilizing the fine particle dispersion, is formed by drawing a resin film containing, for example, iodine and dichromatic pigment (fine particle dispersion) in one direction. The iodine and dichromatic pigment are oriented in a constant direction within the resin film.
An organic compound based linear polarizing film, though it is inexpensive, is fabricated by drawing an organic compound, so that it exhibits a lower durability to heat and abrasion, as compared with an inorganic compound based polarization element.
A polarization element including a fine metal particle dispersion is also known. The fine metal particle dispersion is manufactured, for example, in the following manner. Glass containing halide and silver is heat-treated to deposit and disperse fine silver halide particles in the glass. Subsequently, the glass is heated and drawn to deform the fine silver halide particles into spheroids which are oriented in the direction of the major axis. Next, under a reduction atmosphere, the fine silver halide particles are heated to be reduced to fine silver particles.
However, the polarization element including a fine metal particle dispersion requires a strict temperature management for controlling an aspect ratio (the ratio of the minor axis to the major axis of the spheroids) of the fine silver particles deposited and dispersed in the glass. In addition, resulting polarization element is low in stability of polarization properties.
Specifically, the manufacturing of the foregoing polarization element requires a step of heating fine silver halide particles under a reduction atmosphere to reduce the fine silver halide particles to fine silver particles (hereinafter called a xe2x80x9creduction treatment stepxe2x80x9d) after a step of heating and drawing the fine silver halide particles to provide the same with a shape anisotropy (hereinafter called a xe2x80x9cheating/drawing stepxe2x80x9d). For this reason, the heat applied in the reduction treatment step causes the fine silver particles to again spheroidize. This results in the loss of the shape anisotropy of the fine silver particles, and a deteriorated uniformity of the shape anisotropy (aspect ratio).
Also, when the fine silver halide particles are reduced to fine silver particles in the reduction treatment step, the fine particles are reduced in volume to approximately one-half. This results in a change of the surface of the resulting polarization element into a porous surface. The porous surface scatters incident light to increase the insertion loss of the polarization element. In addition, if moisture in the atmosphere is adsorbed in the porous region of the surface, silver ions are generated. If the polarization element is stained by the silver ions over time, the polarization element fails to provide long-term reliability.
Further, the re-spheroidization of the fine particles will practically limit the processing temperatures in the reduction treatment step. For this reason, the treatment for reducing the fine silver halide particles to the fine silver particles substantially extend to a depth of scores of micrometers from the surface of the glass. This results in residual fine silver halide particles in the glass which do not contribute to the polarization properties. The residual fine silver halide particles increase the insertion loss of the polarization element in addition to the failure in contributing to the polarization properties.
A multi-layer lamination type polarization element is also known. This multi-layer lamination type polarization element is manufactured by using a thin film forming process such as vacuum vapor deposition and sputtering. In this event, several metal layers and dielectric layers are alternately laminated on a glass substrate. Subsequently, the glass substrate is drawn at temperatures higher than the softening point of the glass substrate. At this time, the metal layers are deformed into discontinuous fine metal particle layers oriented in a drawing direction. The polarization properties are obtained by the alternately laminated fine metal particle layers and dielectric layers.
In comparison with the method of manufacturing a polarization element including a fine silver particle dispersion, the method of manufacturing a multi-layer lamination type polarization element is advantageous in terms of the process because of the elimination of the reduction treatment step. However, since the method of manufacturing the multi-layer lamination type polarization element involves the formation of a multi-layer film using a thin film forming process, this method requires a great deal of labor and time, and is not suitable for a reduction in cost. The multi-layer lamination type polarization element is also disadvantageous in that the alternately laminated films are highly susceptible to peeling due to the adhesion of the fine metal particle layers and the dielectric layers.
It is an object of the present invention to provide a polarization element which has high reliability and is easy to manufacture, and a method of manufacturing the same.
To achieve the above object, the method of manufacturing a polarization element of the present invention includes the steps of forming a fine metal particle dispersed product (fine metal particle dispersed film) including a plurality of dispersed fine metal particles on a surface of the transparent substrate using a sol-gel method, drawing the fine metal particle dispersed product together with the transparent substrate with heating to draw the fine metal particle dispersed product and transparent substrate, and cutting the drawn fine metal particle dispersed product and transparent substrate.
The step of forming a fine metal particle dispersed product includes the steps of coating the surface of the transparent substrate with a sol coating liquid, including a disperse liquid containing an organic silicon compound as a main component and a salt of a first metal dispersed in the disperse liquid for generating the fine metal particles, heat-treating the sol coating liquid coated on the transparent substrate or irradiating with an electromagnetic wave the sol coating liquid coated on the transparent substrate, and sintering the heat-treated sol coating liquid coated on the transparent substrate or the sol coating liquid coated on the transparent substrate irradiated with the electromagnetic wave.
Here, the sol-gel method includes the following steps. A sol containing a metal organic or inorganic compound as a main component is solidified through hydrolysis and condensation polymerization reaction into gel. Next, the gel is sintered to produce an inorganic oxide fine metal particle dispersed product such as glass, ceramic or the like.
The fine metal particle dispersed product produced by using the sol-gel method and the transparent substrate are drawn while heated together to deform fine metal particles in the fine metal particle dispersed product so that the fine metal particles have shape anisotropy.
The method of manufacturing the polarization element using the sol-gel method has the following advantages.
(A) a reduction treatment step is not required after the heating/drawing step. This can simplify the manufacturing process and inhibit the fine metal particles from being spheroidized again.
(B) By sintering the sol coating liquid after applying the sol coating liquid with the heat treatment or electromagnetic wave irradiation processing, fine metal particles are substantially completely deposited and dispersed in the matrix material from a metal salt in the sol coating liquid which is a raw material of the fine metal particles. Thus, substantially all fine metal particles contribute to the polarization properties, thereby making it possible to reduce the insertion loss of the polarization element.
(C) By changing the composition of the sol coating liquid, it is possible to control the particle diameters of the fine metal particles dispersed in the matrix material. Specifically, a metal compound of another metal (second metal) different from the fine metal particles, for example, at least one type of metal selected from the group consisting of zirconium, titanium, cerium, tin, bismuth, cobalt, copper, aluminum, magnesium, manganese, chromium, indium, vanadium, iron, nickel, zinc, tungsten, tantalum, hafnium, barium, ytterbium, niobium, molybdenum, yttrium, ruthenium, germanium, lead and boron is blended in the sol coating liquid, thereby making it possible to control the particle diameters of the fine metal particles dispersed in the matrix material.
By the sol-gel method, the metal compound of the second metal also changes into an inorganic oxide (metal oxide) when fabricating a fine metal particle dispersed product which contains an inorganic oxide as a main component. The second metal oxide acts on the particle diameters of the fine metal particles deposited and dispersed in the matrix material. For example, when fine gold particles are dispersed in a matrix material which contains silicon oxide as a main component, a metal compound of copper, cobalt or titanium may be blended in the sol coating liquid such that 0.1 to 50 mass % of metal compound of copper, cobalt or titanium is contained in the fine metal particle dispersed product, permitting the particle diameters of the fine gold particles to be changed in the range of 10 to 100 nm on the average.
The particle diameters of the fine metal particles dispersed in the matrix material affect the aspect ratios of the fine metal particles in the heating/drawing step, so that they significantly affect the polarization properties of the resulting polarization element. Therefore, the shape anisotropy (aspect ratio) of the fine metal particles can be controlled substantially by controlling the particle diameters of the fine metal particles, and therefore the polarization properties can be efficiently controlled.
(D) A multiplicity of layers of laminated fine metal particle dispersed films can be readily formed without using a thin film forming process such as vacuum vapor deposition and sputtering, and adhesion between the laminated films can be improved. It is therefore possible to reduce a cost of a multi-layer film laminated polarization element.
In addition, since the resulting polarization element has the structure comprised of alternately adhered fine metal particle dispersion, and transparent substrates, rather than the structure comprised of alternately adhered fine metal particle layers and dielectric layers, adhesion between the laminated films can be improved. Particularly, when a fine metal particle dispersion made of a matrix material containing silicon oxide as a main component is fabricated on the surface of a transparent substrate such as glass, adhesion between the laminated films can be further improved.